in chemistry, the space in an atom occupied by an electron, a subdivision of the available space within an atom for an electron to orbit the nucleus; also, a mathematical function used to describe the behavior of an electron in an atom

A partial description of the quantum state of an electron (or other particle) orbiting the nucleus of an atom. Different orbitals have different shapes and orientations, depending on the energy of the electron, its angular momentum, and its magnetic number. Orbitals have no clear boundaries; the shape of an orbital, as depicted graphically, shows only the regions around the nucleus in which an electron has a relatively high probability of being found. No more than two electrons (each with opposite spin) can coexist in a single orbital because of the Pauli exclusion principle. See also probability wave, quantum number, shell..